Welcome to the secret world of murky messages! Throughout history, governments, armies, and, of course, spies have used many ways to communicate secretly.

So have schoolchildren. My friends and I used to be devotees of "pig Latin," a spoken "code" designed to baffle fellow students and grown-ups. Here's how: Put the first letter of each word at the end of the word. Then add "ay" to it. So "class" becomes: "lass-cay"; "pencil" is "encil-pay." My friend Bill was "ill-Bay," and so on. We started out speaking slowly, but after some practice, we became fluent and easily understood one another. Others did not.

Sometimes a message can be a code even if you think you understand it. A 1941 radio-broadcast message "East wind rain, north wind cloudy, west wind clear," sounded like a weather report. But it alerted Japanese diplomats around the world that Japan was about to attack the United States at Pearl Harbor.

Other messages are sent by replacing the letters in a word with other letters or symbols. This is called a "cipher" (SIGH-fer). "Meet me after school," might be written: XPPE XPLQ ECDN SZZW, or even 14 5 5 20 14 5 1 6 20 5 18 19 3 8 15 15 12. The code wheel on the facing page is a cipher code. It was perfected in the mid-1400s by the Italian architect Leon Alberti, "the father of Western cryptology."

Cryptology (crip-TOL-uh-gee) is the science of secret communications. Let's take a short tour of its history with David Kahn, a historian and an expert on codes, who points to many instances in which secret messages played a critical role.

In 1556, English spies intercepted messages sent by Mary, Queen of Scots. The coded letters were deciphered by Thomas Phelippes, England's first great cryptologist (code expert). The messages described a plot to kill Queen Elizabeth I. Mary was imprisoned and executed, and England's throne was saved.

But "the most important code solution in history," Mr. Kahn says, was the Zimmermann telegram of World War I. On Jan. 17, 1917, the British intercepted a coded telegram from Germany's foreign minister, Arthur Zimmermann. He was sending instructions to the German ambassador in Mexico. The ambassador was to tell the Mexican government that Germany would help Mexico reclaim its "lost territories" of Texas, New Mexico, and Arizona if it would go to war against the United States. The telegram outraged Americans. A month later, the United States joined the war against Germany.

CODE books are carefully guarded, for obvious reasons. But sometimes a code book falls into enemy hands. In a famous 1914 case, a Russian naval crew recovered German code books from the wreckage of the light cruiser Magdeburg. (Naval code books often had lead covers so that they would sink fast when thrown overboard to avoid capture.) The Allies decoded many messages before the Germans found out and changed codes.

A code machine patented by a Dutch scientist in 1919 was perfected by the Nazis in World War II. The Enigma codemaking machine looked like a portable typewriter. It generated a different series of letters and numbers for the same word in a single message. British codebreakers, aided by research from Polish mathematicians, finally cracked the code.

Today's codemaking machines are much more complex. A computer program called Sherlock has 72 quadrillion possible codes. (A quadrillion is a 1 with 15 zeroes.)

Imagine the possibilities!

To find out more about secret messages, look up "codes," "ciphers," "cryptography," and other such words at your local library. Two especially good books are: "The Codebreakers," by David Kahn (Scribners, 1996) and "Codes, Ciphers, and Secret Writing," by Martin Gardner (Dover, 1984 edition).

For some fictional stories that involve secret codes, try "The Gold Bug," a short story by Edgar Allen Poe; "The Adventure of the Dancing Men," by Sir Arthur Conan Doyle (a Sherlock Holmes story); or "Journey to the Center of the Earth," a science-fiction novel by Jules Verne.